Converters



y 7, 1963 R. H. LEE 3,089,076

CONVERTERS Filed Sept. 8, 1958 A CAT DE NODE p N P N HO 3 /2 FIG.2

FIG. 3

FIG 4 IN V EN TORI ROBERT H. LEE

ATTY

United States Patent Ofifice 3,689,076 Patented May 7, 1963 3,089,076 CONVERTERS Robert H. Lee, Highland, 11h, assignor to Easter Electric (10., Highland, 11h, a corporation of Illinois Filed Sept. 3, 1958, Ser. No. 759,634 3 Claims. (Cl. 321-45) This invention relates to a converter wherein direct current power is converted to alternating current power, and this application is a continuation-in-part of my copending application, Serial No. 733,975, filed May 8, 1958.

It is a primary object of the present invention to provide a converter which is capable of efficiently driving a load having either a leading or lagging power factor and in which there is relatively little loss of power within the converter.

It is also an object of the present invention to provide a converter of the type stated which is particularly useful in delivering alternating current power at a relatively constant frequency to a load, the impedance of which is a function of frequency.

It is another object of the present invention to provide a converter of the type stated which utilizes solid-state, controlled, semiconductor rectifiers in the switching mode, said rectifiers having thyratron-like characteristics.

It is an additional object of the present invention to provide a converter of the type stated wherein the rectifiers are protected from damage due to transient voltages in excess of their rated capacity.

With the above and other objects in view, my inven tion resides in the novel features of form, construction, arrangement, and combination of parts presently described and pointed out in the claims.

In the accompanying drawing (one sheet) FIG. 1 is a circuit diagram showing a converter constructed in accordance with and embodying the present invention;

FIG. 2 is a schematic illustration of one of the rectifiers used in the converter of the present invention;

FIG. 3 is a diagram illustrating the Wave forms of the voltage across the primary winding of the power transformer of the converter and the current through the load; and

FIG. 4 is a diagram illustrating the current fiow through the source of direct current power.

Referring now in more detail and by reference characters to the drawing, which illustrates the preferred embodiment of the present invention, FIG. 1 illustrates a converter comprising a pair of solid-state, silicon, controlled rectifiers 1, 2, each being of the type described in the article by R. P. Frenzel and F. W. Gutzwiller, entitled Solid-State Thyratron Switches Kilowatts (Electronics, March 28, 1958). These rectifiers 1, 2, are of the P-N-P-N construction as shown in FIG. 2 and include an anode and cathode terminal plus a control gate which is connected to the center P. The control gate switches the rectifier from the non-conducting to the conducting state when a critical positive gate-to-cathode current is reached at which time anode-to-cathode voltage becomes very low and current flowing from anode-to-cathode is essentially unchanged by variations in gate current. Thus, the rectifiers 1, 2 are semiconductor switches which possess characteristics similar to those of thyratrons except that in the rectifiers 1, 2 conduction is controlled by positive gate-to-cathode current while in thyratrons the firing usually is controlled by a critical positive grid-tocathode voltage.

The rectifiers 1, 2, each have anodes 3, 4, cathodes 5, 6, and gates 7, 8. The cathodes 5, 6 are connected to opposite ends, respectively, of the primary winding 9 of a power transformer 1t), and the anodes 3, 4 are connected to the positive terminal 11 of a battery or other suitable source of direct current power 12. The negative terminal 13 of the battery 12 is connected to the center tap 14 of the primary winding 9. Connected across the anode 3 and cathode 5 is a diode 15, and similarly connected across the anode 4 and cathode 6 is a second diode 16. As will be apparent from FIG. 1, the diodes 15, 16 are connected in a current-blocking direction with respect to the normal direction of current flow from the battery 12 through the rectifiers 1, 2.

Connected to the gates 7, 8 for supplying switching or trigger-signal current thereto are signal generators 17, 18, which alternately switch the rectifiers 1, 2 from the nonconducting to the conducting state. Thus, when the rectifier 1 is biased to cause conduction, the rectifier 2 will not conduct since the anode voltage and current is reduced to zero by the signal generator 18, and, similarly, when the rectifier 2 is biased to allow conduction, the rectifier 1 will not conduct since its anode voltage and current is reduced to zero by the signal generator 17. As a result, the rectifiers 1, 2 are utilized in the switching mode to supply current from the battery 12 alternately to each half of the primary winding 9. The signal generators 17, 18 may be any suitable source of signal, as, for example, a square wave oscillator, in which case the signal generators 17, 18 each represent one-half of the secondary winding of the output transformer of such oscillator with the ends of the secondary winding connected to the gates 7, 8, and the center tap connected to the battery terminal 11. If desired, diodes 19, 20 may be inserted in the gate-to-anode circuits so as to limit the value of reverse gate current which might otherwise damage or modify the operating characteristics of the rectifiers 1, 2.

The load 21 is connected to the secondary winding 22 of the transformer 10, and the load 21 may be of any type of resistive or reactive load, but, in most situations, the load will have a power factor since most A.C. loads are not purely resistive in nature. For example, the load may constitute a motor, the impedance and power factor of which varies according to the mechanical load thereon. The load 21 might also consist of a fluorescent lighting circuit which normally has a relatively low power factor, but such power factor may be either leading or lagging, depending upon the ballast used. However, in power factor loads, a component of current from the load 21 will flow that is not in phase with the load voltage. Therefore, during some time period in each half cycle of operation, a current must flow in a direction opposite to the normal current flow through the rectifiers 1, 2. The rectifiers 1, 2 will, of course, block this reverse flow of current, but the diodes 15, 16 will provide a path through which this reverse current will fiow back to and in a direction to charge the battery 12 during a portion of each half cycle of operation.

In connection with the present invention, it has also been found that since it is impossible to provide perfect coupling between the two halves of the primary winding 9, when current flows through one-half of the primary winding 9, and momentarily thereafter the switching circuit provided by the rectifiers 1, 2 and switching-signal generators 17, 18 causes current to flow in the other half of the primary winding, the switching is not instantaneous, with the result that the residual current in the one-half of the primary winding 9 will cause transient voltages to build up across the rectifier which had been carrying the current during the switching interval. Under certain conditions, these transient voltages might exceed rated voltages and thus damage the rectifiers 1, 2. However, the diodes 15, 16 provide a current fiow path and thus prevent build-up of excess transient voltages across the rectifiers 1, 2 during the switching interval.

Referring now to FIGS. 3 and 4, FIG. 3 shows a curve 23 which represents the wave form of the voltage across the primary winding 9, while the curve 24 represents the Wave form of the current passing through the load 21, assuming the latter is a purely inductive load. Furthermore, in plotting the curves 23, 24, it has been assumed that the load 21 has a lagging power factor, that no power is consumed in the load, and that there are no power losses in the converter. In FIG. 4, the curve 25 shows the wave form of the current flow through the battery 12, also assuming no power losses. It will be seen that the current through the battery is positive for about on-half of the time and negative for the remaining half of the time, thus illustrating that in a power factor load 21, the reverse current passing through the diodes 15, 16, will, during a portion of the cycle, supply current to and charge the battery 12.

It will be apparent that the converter will operate efiiciently to drive power factor loads and, due to the presence of the diodes 15, 16, the reverse current reflected back to the primary winding 9 will be usefully employed in returning energy to the battery :12. Converters constructed in accordance with the present invention are particularly useful wherein alternating current power of relatively constant frequency is desired in the load circuit. For example, there are many items of electrical equipment which are designed to operate satisfactorily at a nominal frequency of 60 cycles per second, but ordinarily such equipment will operate within a limited frequency range, such as 50 to 100 cycles per second. However, the converter may be used wherein the desired frequency of operation may vary over a relatively wide range.

It should be understood that changes and modifications in the form, construction, arrangement, and combination of the several parts of the converters may be made and substituted for those herein shown and described without departing from the nature and principle of my invention.

Having thus described my invention, what I claim and desire to secure by Letters Patent is:

1. In combination with a power factor A.C. load, a power supply device for converting direct current to alternating current comprising a direct current source having first and second terminals, a transformer having a primary winding and a secondary winding, said primary winding having two end terminals and an intermediate terminal tapped 08? between such end terminals whereby to establish a primary circuit having two branches, said intermediate terminal being connected to the first terminal of the direct current source, said secondary winding having output terminals to which the AC. load is connected so that a component of current from the load which is not in phase with the load voltage will flow alternately in the two branches of the primary circuit, first and second transistors having thyratron-like characteristics, each of said transistors including a gate, a cathode and an anode, two signal generating means each having two terminals, one terminal of one signal generating means being connected to one terminal of the other signal generating means by a common lead which is also connected to the second terminal of the direct current source, the other terminals of the two signal generating means being respectively connected to the gates of the two transistors whereby to alternately bias said transistors off and on so that during one-time-interval the first transistor is conducting while the second transistor is nonconducting and during a next succeeding time-interval the second transistor is conducting while the first transistor is non-conducting, the cathodes of the two transistors being respectively connected to the end terminals of said primary Winding, the anodes of the two transistors both being connected to the common lead between the two signal generating means, a first diode shunted across the cathode and anode of the first transistor thereby being connected between one end terminal of the primary winding and the second terminal of the direct current source to provide a unidirectional flow-path for transmitting the component of current from the load which flows from such end terminal to the direct current source during the time-interval when the first transistor is non-conducting whereby to return energy to said direct current source during the off-cycle of the first transistor, a second diode shunted across the cathode and anode of the second transistor and thereby being connected between the other end terminal of the primary winding and the second terminal of the direct current source to provide a unidirectional flow-path for transmitting the component of current from the load which flows from such end terminal to the direct current source during the time-intervals when the first transistor is non-conducting whereby to return energy to said direct current source during the off-cycle of the second transistor, a third diode connected between the gate and anode of the first transistor to provide a unidirectional flow-path for limiting any reverse gate current which may tend to flow from the anode to the gate of said first transistor, and a fourth diode connected between the gate and anode of the second transistor to provide a unidirectional flowpath for limiting any reverse gate current which may tend to flow from the anode to the gate of said second transistor.

2. A power supply device for converting direct current to alternating current according to claim 1 in which the intermediate terminal of the primary winding is a center tap which divides the primary into two substantially equal parts.

3. A power supply device for converting direct current to alternating current according to claim 1 in which the direct current source is a battery, the first terminal of such direct current source is negative and the second terminal of such direct current source is positive.

References Cited in the file of this patent UNITED STATES PATENTS 2,443,100 Edwards June 8, 1948 2,504,834 Hartwig Apr. 18, 1950 2,604,612 Rudolph July 22, 1952 2,644,895 Lo July 7, 1953 2,722,649 Immel et al. Nov. 1, 1955 2,783,384 Bright et al. Feb. 26, 1957 2,884,545 Houck Apr. 28, 1959 2,888,622 Mooers May 26, 1959 2,965,833 Jensen Dec. 20, 1960 FOREIGN PATENTS 665,071 Germany Sept. 17, 1938 OTHER REFERENCES Solid-State Thyratron Switches Kilowatts, Frenzel et al.; Electronics; March 28, 1958. 

1. IN COMBINATION WITH A POWER FACTOR A.C. LOAD, A POWER SUPPLY DEVICE FOR CONVERTING DIRECT CURRENT TO ALTERNATING CURRENT COMPRISING A DIRECT CURRENT SOURCE HAVING FIRST AND SECOND TERMINALS, A TRANSFORMER HAVING A PRIMARY WINDING AND A SECONDARY WINDING, SAID PRIMARY WINDING HAVING TWO END TERMINALS AND AN INTERMEDIATE TERMINAL TAPPED OFF BETWEEN SUCH END TERMINALS WHEREBY TO ESTABLISH A PRIMARY CIRCUIT HAVING TWO BRANCHES, SAID INTERMEDIATE TERMINAL BEING CONNECTED TO THE FIRST TERMINAL OF THE DIRECT CURRENT SOURCE, SAID SECONDARY WINDING HAVING OUTPUT TERMINALS TO WHICH THE A.C. LOAD IS CONNECTED SO THAT A COMPONENT OF CURRENT FROM THE LOAD WHICH IS NOT IN PHASE WITH THE LOAD VOLTAGE WILL FLOW ALTERNATELY IN THE TWO BRANCHES OF THE PRIMARY CIRCUIT, FIRST AND SECOND TRANSISTORS HAVING THYRATRON-LIKE CHARACTERISTICS, EACH OF SAID TRANSISTORS INCLUDING A GATE, A CATHODE AND AN ANODE, TWO SIGNAL GENERATING MEANS EACH HAVING TWO TERNINALS, ONE TERMINAL OF ONE SIGNAL GERERATING MEANS BEING CONNECTED TO ONE TERMINAL OF THE OTHER SIGNAL GENERATING MEANS BY A COMMON LEAD WHICH IS ALSO CONNECTED TO THE SECOND TERMINAL OF THE DIRECT CURRENT SOURCE, THE OTHER TERMINALS OF THE TWO SIGNAL GENERATING MEANS BEING RESPECTIVELY CONNECTED TO THE GATES OF THE TWO TRANSISTORS WHEREBY TO ALTERNATELY BIAS SAID TRANSISTORS OFF AND ON SO THAT DURING ONE-TIME-INTERVAL THE FIRST TRANSISTOR IS CONDUCTING WHILE THE SECOND TRANSISTOR IS NONCONDUCTING AND DURING A NEXT SUCCEEDING TIME-INTERVAL THE SECOND TRANSISTOR IS CONDUCTING WHILE THE FIRST TRANSISTOR IS NON-CONDUCTING, THE CATHODES OF THE TWO TRANSISTORS BEING RESPECTIVELY CONNECTED TO THE END TERMINALS OF SAID PRIMARY WINDING, THE ANODES OF THE TWO TRANSISTORS BOTH BEING CONNECTED TO THE COMMON LEAD BETWEEN THE TWO SIGNAL GENERATING MEANS, A FIRST DIODE SHUNTED ACROSS THE CATHODE AND ANODE OF THE FIRST TRANSISTOR THEREBY BEING CONNECTED BETWEEN ONE END TERMINAL OF THE PRIMARY WINDING AND THE SECOND TERMINAL OF THE DIRECT CUR- 